There are some events in history about which there can be no ifs or buts. The Latvian volunteer militias in 1941 were so zealous in murdering Jews that by 1943 they had not only killed nearly all of Latvia's 70,000 Jewish community, they had also slaughtered 20,000 Jews from central Europe who had been deported to Riga, and tens of thousands of Jews in Belarus. The veterans of these volunteer units formed the backbone of the 15th and 19th Latvian Waffen-SS divisions. The annual unofficial parade in Riga honouring these veterans is the national disgrace of an independent Latvia, and an international stain on Nato and the EU to which the Baltic state now belongs. No ifs, no buts.

Latvian participation in the Holocaust differs from the massacre of the 300 Jewish men, women and children in Jedwabne, Poland in 1941 in important respects – notably the Poles acted largely alone. Nor can there be any moral equivalence between the murder of 4 million Jews in Poland under Nazi occupation and the collusion of some Jewish leaders in eastern Poland with the Soviet occupation between 1939 and 1941. And yet this is precisely the point that the man who leads British conservatives in the European parliament seeks to make.

Michal Kaminski denies everything, of course. His membership of the neo-fascist National Rebirth of Poland? Just a youthful fling. The fact that he did indeed wear a fascist symbol called the Chrobry Sword, having said he had never heard of the emblem? Apparently that denial, in an interview with the Jewish Chronicle, was a result of a "mispronunciation". But Mr Kaminski has made his position on Jedwabne crystal clear. He opposed the apology for the massacre made by the then president of Poland on behalf of the Polish people. Mr Kaminski said: "If you are asking the Polish nation to apologise for the crime made in Jedwabne, you would require from the whole Jewish nation to apologise for what some Jewish communists did in eastern Poland." Mr Kaminski's aim is to establish an equivalence between communism and Nazism, casting Jews as the leading players in communism and the Soviet occupation. This rests on two discredited ideas: that the Soviet occupation was even remotely comparable with the Holocaust, and that Bolshevism was a Jewish phenomenon.

The Latvian Fatherland and Freedom party is playing similar tricks with history. In attending the annual parade of Waffen-SS veterans, it says it is doing no more than any other Latvian party. In fact, the party is not merely the most enthusiastic supporter of these events. It was also the sponsor of a law two years ago to declare the Waffen-SS legionnaires a national resistance movement, which would entitle surviving veterans to military pensions. This is denied to those Latvians who fought for the allies against Hitler. The law failed because other Latvian parties opposed it.

Colin Powell could see it. He told Latvia during the Nato enlargement talks that the Nazi rallies had to stop, after which the rallies stopped being sanctioned by the state, but continued as unofficial events. So if a Republican US secretary of state can grasp a simple truth about Latvia's past, why can't David Cameron, Eric Pickles and William Hague?

This issue will not die a media death. It will resurface every time Mr Kaminski gets to his feet in the European parliament. It will haunt Mr Cameron every time the Tories open their mouths about the EU, an organisation fashioned to make sure Europe never again repeats the events of 1939-45. Beyond the events themselves, the arguments the Tories are marshalling in defence, especially in the Latvian case, are troubling. They were conscripts, they were faced with Hobson's choice, they were only fighting for their country. Sound familiar? Is Holocaust revisionism really part of Mr Cameron's vision of modern conservatism?